1. Field of the Invention
The present invention relates to a power inverting apparatus for changing ac voltage into dc voltage and back into ac voltage.
2. Description of the Related Art
To begin with, a basic configuration and operation of major circuits of a power inverting apparatus will be described in conjunction with the first prior art shown in FIGS. 63 and 64. FIG. 63 is a block diagram showing a constant-voltage/constant-frequency power supply disclosed in, for example, Japanese Patent Laid-Open No. 2-231965. FIG. 64 is an explanatory diagram showing the operation of the power supply. In FIG. 63, A denotes a converter made by connecting diodes 1 and 2 serving as rectifying elements in series with each other. Adc output terminal of the converter A is connected to a smoothing capacitor 12.
Reference alphabet B denotes a selector. C denotes an inverter. The selector B or inverter C consists of two switching circuits 3 and 4 or 5 and 6. The switching circuit 3 is composed of a switching element 3t that is, for example, a transistor or a field-effect transistor (hereinafter, FET), and a flywheel diode 3d connected in parallel with but in the opposite direction of the switching element 3t. The same applies to the switching circuits 4, 5, and 6.
The input and output terminals of the converter A, smoothing capacitor 12, selector B, and inverter C are connected in parallel with one another over connection lines 10 and 11.
Reference numeral 7 denotes an ac voltage source for supplying an input voltage Vs. One of the terminals of the ac voltage source 7 is connected to a middle point of the converter A via a connection line 8. The other terminal thereof is linked with the connection line 9. 14 denotes a loading circuit, one of whose terminals is connected to a middle point of the inverter circuit C; that is, one ac output terminal via the connection line 13. The other terminal of the loading circuit 14 is connected to a middle point of the selector B; that is, the other ac output terminal via the connection line 9.
Next, the operation of the foregoing conventional power supply will be described. FIGS. 64a to 64e show various states of switching elements and current flows. First, when the converter operates, current flows as indicated in FIG. 64b within a range defined with a wave A of voltage shown in FIG. 64a or during a positive half cycle thereof. Within a range defined with a wave B of the voltage shown in FIG. 64a or during a negative half cycle thereof, current flows as indicated in FIG. 64d. Thus, the smoothing capacitor 12 is charged with the peak-to-peak value of the input voltage. A dc voltage V.sub.D shown in FIG. 65 is provided as a product of the square root of 2 multiplied by Vs.
The inverter circuit C operates as described below. For outputting the positive voltage (range A) shown in FIG. 64a, the switching elements 4t and 5t shown in FIG. 64c are turned on. For outputting the negative voltage (range B) shown in FIG. 64a, the switching elements 3t and 6t are turned on. Thus, both the positive and negative voltages can be supplied. If pulse-width modulation (PWM) is controlled at either of the switching elements in each of FIGS. 64c and 64e, voltage having a sine wave is made available.
To provide the positive voltage within a hatched range in FIG. 65, the switching element 4t in the selector B must be conducting and the switching element 5t in the inverter C must control pulse-width modulation according to a sine-wave command. To provide the negative voltage within a hatched range in FIG. 65, the switching element 3t in the selector B must be conducting and the switching element 6t in the inverter C must control pulse-width modulation according to a sine-wave command.
Under the foregoing switching control, when an inverter output voltage V.sub.INV, which is not shown, is passed through a filter circuit, a sine-wave voltage like a negative voltage V.sub.L shown in FIG. 65 is made available. Herein, the input voltage Vs and output voltage V.sub.L may be different in waveform from each other but must be in phase with each other (their zero points must coincide with one another).
Next, a constant-voltage/constant-frequency power supply of the second prior art will be described concerning mainly a control relationship using FIG. 66.
The second prior art adopts a converter D instead of the converter A in FIG. 63.
The converter D is made by connecting switching circuits 1 and 2 in series with each other. The switching circuit 1 or 2 consists of a switching element 1t or 2t that is a transistor or an FET, and a diode 1d or 2d.
Reference numeral 15 denotes an ac reactor. One terminal of the ac reactor 15 is connected to a middle point of the converter D via a connection line 8, and the other terminal thereof is connected to the ac voltage source 7 via a connection line 16.
Reference numeral 30 denotes a control circuit for controlling switching elements it to 6t included in the converter D, selector B, and inverter C. 100 denotes a converter voltage command generator for actuating the converter with a high power factor. 101 denotes a carrier generator. 102 denotes an inverter voltage command generator for use in reshaping load voltage in the form of a sine wave. 103 denotes a comparator For comparing an output V.sub.CMD1 of the converter voltage command generator 100 with an output CAR of the carrier generator 101. 104 denotes a comparator for comparing an output V.sub.CMD2 of the inverter voltage command generator 102 with an output CAR of the carrier generator 101. 105 denotes a zero-crossing detector for detecting a zero crossing by analyzing an output of the inverter voltage command generator 102. 106 denotes a switching pattern generator for generating switching signals T.sub.1 to T.sub.6 using the outputs of the comparators 103 and 104 and the zero-crossing detector 105. FIG. 67 shows a circuitry of the switching pattern generator 106. 107 to 109 denote reversing circuits. 110, 111, 113, and 114 denote AND circuits. 112 and 115 denote OR circuits. 116 and 117 denote reversing circuits.
Other circuit elements except the foregoing ones are identical to those in the first prior art described previously.
Next, the operation of the second conventional power supply will be described with reference to the timing chart of FIG. 68. The comparator 103 compares the converter voltage command V.sub.CMD1 provided by the converter voltage command generator 100 with the carrier CAR provided by the carrier generator 101. When the carrier CAR is smaller, the comparator 103 outputs a converter PWM signal Ta having a high logic level. Likewise, the comparator 104 compares the inverter voltage command V.sub.CMD2 provided by the inverter voltage command generator 102 with the carrier CAR, and then outputs an Inverter PWM signal Tb. When the inverter voltage command V.sub.CMD2 is positive, the zero-crossing detector 105 outputs a polarity signal Tc having a high logic level.
In FIG. 67, the polarity signal Tc is used as a switching signal T4, and a signal that is the polarity signal Tc reversed in polarity by the reversing circuit 107 is used as a switching signal T3. The AND circuits 110 and 111 and the OR circuit 112 constitute a selector. When the polarity signal Tc is high, the OR circuit 112 serving as the output terminal of the selector outputs the converter PWM signal Ta. When the polarity signal Tc is low, a signal that is the converter PWM signal Ta reversed polarity by the reversing circuit 108 is output. The output signal is used as a switching signal T1, and a signal that is opposite in polarity to the switching signal T1 and provided by the reversing circuit 116 is used as a switching signal T2. Likewise, the AND circuits 113 and 114 and the OR circuit 115 constitute a selector. When the polarity signal Tc is high, the OR circuit 115 serving as the output terminal of the selector outputs the Inverter PWM signal Tb. When the polarity signal Tc is low, the OR circuit 115 outputs a signal that is opposite in polarity to the inverter PWM signal Tb and provided by the reversing circuit 109. The output signal is used as a switching signal T5, and a signal that is opposite in polarity to the switching signal T5 and provided by the reversing circuit 117 is used as a switching signal T6.
When two circuits of the converter D and selector B are discussed together, it will be understood that the circuits acts as a typical converter for changing ac voltage supplied by the ac voltage source 7 into dc voltage to charge the smoothing capacitor 12. The switching pattern of the converter D is consistent with the waveform of the converter PWM signal such as the one indicated as T1 or T2 in FIG. 68.
Assuming that the converter voltage command is substantially in phase with the inverter voltage command, the switching pattern of the selector B becomes consistent with the waveform of the converter command V.sub.CMD1. Consequently, the converter output voltage V.sub.CNV shown in FIG. 68 is provided.
When two circuits of the inverter C and selector B are discussed together, it will be understood that the circuits act as a typical inverter for changing de voltage existent at the smoothing capacitor 12 into ac voltage and then outputting the ac voltage. At this time, the switching pattern of the inverter C is consistent with the waveform of the Inverter PWM signal such as the one indicated as T5 or T6 in FIG. 68. The switching pattern of the selector B is consistent with the waveform of the polarity signal Tc For the inverter voltage command V.sub.CMD2. Consequently, the inverter output voltage V.sub.INV shown in FIG. 68 is provided.
The conventional constant-voltage/constant-frequency power supplies are configured as mentioned above. That is to say, the selector B is shared between a converter and an inverter. The selector B can therefore operate only under the condition that both the converter D and inverter C operate with the same polarity. For supplying stable voltage to a load, since the switching pattern of the inverter C cannot be altered, the selector B cannot help adopting a switching pattern dedicated to an inverter. If input voltage is abruptly reversed in polarity, the converter D Fails to operate correctly.
For example, when an uninterruptible power supply (CVCF) is concerned, a generator may be included in an input system. In this case, an abrupt phase change may occur due to switching between the power supply and generator. When the inverter C outputs positive voltage, if the input voltage is reversed 180.degree. to be negative, since the selector B shown in FIG. 66 turns on the switching element 4t in order to provide inverter output voltage of positive polarity, the reactor 15 alone is connected from the ac voltage source 7 to the selector B via the switching element 4t and the diode 2d of the switching circuit 2 in the converter D. If this state continues for more than several milliseconds, overcurrent flows internally. The switching element 4t is therefore turned off forcibly. A desired inverter voltage cannot be output.